(1) Field of the Invention
This invention relates to a brushless d.c. motor with one position sensor, particularly to a brushless d.c. motor having one position sensor and useful for use as a thin disk-shaped brushless d.c. fan motor (i.e., brushless d.c. axial-flow fan). The term "d.c." as used herein should be interpreted in such a broader sense that it covers not only direct currents (d.c.) but also single-phase currents. For the sake of simplification, the invention will hereinafter be described using "d.c." only.
(2) Description of the Prior Art
Reflecting the development of a variety of apparatus in recent years, there is a strong demand for brushless motors suitable for such apparatus, especially, brushless d.c. motors. Since such brushless d.c. motors are also useful as brushless fan motors for office machines, there is a demand, depending on machines to be incorporated therein, toward brushless fan motors which are extremely economical, small, extremely-thin and flattened, and light, apart from efficiency of rotation (needless to say, they would be useless unless they have efficiencies of rotation above a certain level).
Use of a conventional brushless motor for the construction of brushless fan motor is however difficult to provide a disk-shaped brushless d.c. fan motor having a thickness as small as 10-15 mm, because such a conventional brushless motor is of the cylindrical and core-equipped type and electrical parts for its drive circuit, including a position sensor, must also be assembled in the brushless motor. Owing to the core-equipped nature of the brushless motor, it is inherently heavy and cannot thus be constructed into any model having an extremely light weight. These brushless fan motors were thus accompanied by such drawbacks that certain types of housings did not permit their incorporation and otherwise needed some changes to their designs. Due to their core-equipped structures, they were formed of many parts and they required expensive equipment for winding armature coils on cores. In addition to these shortcomings, they were not able to provide excellent productivity.
As a brushless motor capable of satisfying the above-mentioned requirements to a highest possible degree, a disk-shaped brushless and coreless d.c. motor having one armature coil and one position sensor may then be contemplated. Such a disk-shaped brushless d.c. motor cannot however be caused to rotate continuously unless certain special means is applied thereto, although it may be possible to rotate its rotor magnet over a certain predetermined range. Therefore, it was not able to make up any disk-shaped brushless motor. Even if a motor equipped with only one armature coil and position sensor should be able to rotate, it is unexpectable to obtain any large rotary force with such a single piece of armature coil. For large rotary forces, it is indispensable to use two or more armature coils.
When designing, for example, a disk-shaped brushless motor having two armature coils as stator armatures, it has conventionally been necessary to use two position sensors. Namely, it has been required, for permitting continuous rotation, to design such a disk-shaped brushless motor into two-phase disk-shaped brushless motor which require two position sensor. Magnetoelectric transducers such as Hall elements or Hall IC devices are often used as position sensors. These position sensors are however costly. It is certainly preferred from the viewpoint of mass production of economical, small and disk-shaped brushless motors, especially, disk-shaped brushless fan motors if each of such motors can be constructed with a single piece of position sensor.
However, use of a single piece of position sensor is accompanied by such a problem that similar to the above-mentioned motor with a single piece of coil, the resultant motor cannot start by itself when the position sensor detects the boundary area between an N pole and its matching S pole of the rotor magnet (i.e., field magnet), namely, the dead point at the time of its start. Namely, the torque of a brushless d.c. motor reaches zero at points where the current is switched over. In other words, the brushless d.c. motor contains so-called "dead points", which lead to the drawback that the motor cannot start by itself when the rotor is located by chance at either one of such dead points at the time of its start.
In the case of a disk-shaped brushless d.c. motor, it is therefore allowed to overcome such dead points and to make a self-start even with a single piece of position sensor by applying a torque (cogging torque) from a cogging-generating magnetic member (an iron piece is used) in addition to torques produced by its armature coils and rotor magnet.
As an exemplary method for applying a cogging torque in a brushless motor of the coreless type, it may be possible to dispose iron members, which produce cogging torques, within the air gap of a space magnetic field as shown in FIG. 13 or to form tilted projections on a stator yoke as illustrated in FIG. 14. In FIGS. 13 and 14, there are depicted rotor yokes 50, 6-pole magnet rotors 51 with alternating N and S magnetic poles, air-core type armature coils 52, air gaps 53, stator yokes 54 and iron rods 55. As a method for applying cogging in a coreless motor, the iron rod 55 may be inserted partly in the air gap 53 having a constant width as shown in FIG. 13. According to this method, a magnetic flux 56 is produced as depicted in FIG. 15, whereby causing the magnet rotor 51 to stop at a point where the center of the N or S magnetic pole of the rotor magnet 51 confronts the iron rod 55. Thus, a self-starting brushless motor of the coreless type can be obtained provided that the armature coils 52 are arranged at such points as permitting the production of rotary torques at such a stopped point. The method shown in FIG. 13 is however accompanied by such a drawback that if the thickness of each iron rod 55 is increased to make the cogging torque greater, the magnetic flux 56 is applied in the vicinity of the dead point as depicted in FIG. 16 and the torque is thus lowered near the dead point. This method also requires many parts, thereby making the assembly work difficult. Besides, unless optimum members are chosen as the iron members, it is impossible to obtain an ideal composite torque curve which is in turn obtained from ideal cogging torques and ideal armature coil torques. On the other hand, it has also been known to make the air gap 54 of the space magnetic field tilted as shown in FIG. 14. This method is however accompanied by such drawbacks that the magnetic flux density is lowered, large rotary torques cannot be obtained and the efficiency is hence lowered since the width of the air gap of the space magnetic field increases. In addition, both of the above methods require such a structure that the stator yoke is provided uniformly in the entire plane which lies in opposition to the magnet rotor. Due to the provision of this stator yoke, there is no space sufficient for easy arrangement of the position sensor and electrical parts. It was therefore not possible to construct with ease a brushless d.c. motor, disk-shaped brushless d.c. motor or especially a disk-shaped brushless d.c. fan motor, which had a thickness as small as 10-15 mm. Particularly when transistors, resistors, etc. are used as electrical parts, they may be put together into a compact form if a chip-type part is employed. Such chip-type parts are however expensive. For models requiring low manufacturing costs, transistors and the like must be relied upon in spite of their large sizes. Use of IC devices as drive circuits results in compact electrical parts. However, it is necessary to use very expensive facilities to obtain IC devices which are commonly applicable to various models. It was thus tried to use commercially-available IC devices. However, such commercial IC devices were not applicable to some models due to their shapes, sizes, etc. These are drawbacks of IC devices.
In order to obtain an ideal torque-turning angle curve, it is indispensable to obtain such a composite torque curve 57 as shown in FIG. 17. Numeral 58 indicates an armature coil torque curve obtained by an armature coil. Designated at numeral 59 is a cogging torque curve obtained by a cogging-producing magnetic member. As apparent from the armature coil torque curve 58 and cogging torque curve 59, it is necessary to make the magnitude of the cogging torque one half of that of the armature torque. In this manner, it is possible to obtain, as a combination of the armature coil torque and cogging torque, the composite torque curve 57 which is substantially constant over the entire range of the turning angle. It is therefore necessary to design a self-starting brushless d.c. motor in such a way that such an ideal composite torque curve as that 57 illustrated in FIG. 17 is obtained.
Although the above description has been made primarily with respect to disk-shaped brushless d.c. motors, the same description also applies equally to cylindrical brushless d.c. motors.